1. Field of the Invention
This invention relates to a color image processing apparatus and a color printer system and in particular to a color image processing apparatus and a color printer system for generating image signals of four colors of cyan (C), magenta (M), yellow (Y), and black (K) from red (R), green (G), and blue (B) image signals.
2. Description of the Related Art
Although the basic primary colors in color print are three colors of cyan (C), magenta (M), and yellow (Y), a printer with four colors of C, M, Y and black (K) as the basic primary colors is often used from the viewpoint of the quality of frequently used black font, the print cost, or the advantage in a clear natural image.
On the other hand, input image data of a business printer, etc., used in an office generally is provided by three-color data of red (R), green (G), and blue (B) and therefore four-color separation of a process of generating CMYK data from RGB data is required.
Usually the four-color separation is implemented as processing of calculating the complement of RGB data (r, g, b), generating CMY data (c, m, y), and partially replacing CMY common element min {c, m, y} with K component data (k) where min {c, m, y} is the minimum value of the CMY data (c, m, y), as disclosed on page 476 of “PostScript Language Reference third edition” (Adobe Systems Incorporated, February 1999).
The K component generated by performing the processing as described above is called BG (Black Generation) and the CMY component removed as the CMY data is replaced with K is called UCR (Under Color Removal).
The effect of decreasing the maximum color material amount per unit print area is shown as one of the large merits of such BG processing and UCR processing. In the Specification, the maximum color material amount per unit print area will be hereinafter called simply the maximum color material amount or if the printer is a laser printer, the maximum toner amount.
For example, a method called 100% UCR is available as a method capable of most decreasing the maximum color material amount. This is a method of subtracting k found as k=min {c, m, y} from CMY data (c, m, y) to obtain CMY data (c′, m′, y′) after undergoing the UCR processing as in expression (1):
                                                                                          c                                                                                                    ⁢                    ′                                                  ⁢                                                                  =                                                                  ⁢                                  c                  ⁢                                                                          -                                                                          ⁢                  k                                                                                                                          m                                                                                                    ⁢                    ′                                                  ⁢                                                                  =                                                                  ⁢                                  m                  ⁢                                                                          -                                                                          ⁢                  k                                                                                                                          y                                                                                                    ⁢                    ′                                                  ⁢                                                                  =                                                                  ⁢                                  y                  ⁢                                                                          -                                                                          ⁢                  k                                                                    }                            (        1        )            
The maximum color material amount of four-color print of CMYK in the 100% UCR becomes 200% of the maximum color material amount per color. Since the maximum color material amount to print in four colors of CMYK without performing UCR processing is 400%, it is seen that the maximum color material amount can be adjusted in the range of 200% to 400% by performing BG processing and UCR processing.
On the other hand, generally as for printers for printing in multiple colors, it is known that the more excessive the maximum color material amount, the larger the apparatus load, resulting in a tendency toward the instability of reproducibility of overlaid colors.
For example, in a laser printer, as the toner layer is thicker, the transfer performance becomes more easily unstable and in an ink jet printer, as the ink amount is larger, ink bleeding to paper becomes easier to occur. In a laser printer for fusing toner, if an attempt is made to cover the maximum toner amount, the heat energy to be input in a unit time becomes large and thus problems of upsizing of a fuser and shortening of the fuser life are introduced.
Thus, to develop a printer engine, requirement for cutting down the maximum color material amount as long as the image quality is allowed occurs. However, from the viewpoint of the image quality, if extreme UCR processing like 100% UCR is performed, an extremely hard and dark image low in saturation on the whole is produced and the contrast is degraded because the color material amount is small.
From the contrary demands, the value to provide the best balance between the engine performance and the image quality is selected for design in the range of about 300% to 350% per color for commercial print or in the range of about 200% to 300% in a popularly priced laser printer, etc., for limitation of the maximum color material amount.
On the other hand, an art of attaching a density sensor to a printer and making gray level correction using a detection signal from the sensor, thereby controlling the color material amount is also already known.
For example, JP-A-10-39555 discloses an art of optimizing a look-up table (LUT) to convert the density data for each gray level into an output signal by performing first gray level control and controlling the toner replenishment amount for stabilizing the image density by performing second gray level control to keep the gray level characteristic constant against environment fluctuation. However, this example concerns stabilization of the single-color gray level characteristic and does not provide means for circumventing image instability caused by excess of the total toner amount in color print as described above.
JP-A-5-328131 discloses an art of managing the toner deposition amount for adjusting the density constant so that the density of the whole output image does not change if the user adjusts the gray level characteristic.
Further, JP-A-2001-255711 proposes an art of calculating the toner deposition amount of an image based on the gray level data subjected to gray level correction and measurement data of a temperature-humidity sensor and alerting the user by notification means if the calculation result is equal to or greater than the upper limit value of the toner deposition amount determined by the fixing performance of a fuser.
The arts in JP-A-5-328131 and JP-A-2001-255711 assume that hardware feedback is executed in response to user's adjustment. Thus, in a printer shared among computers, a sequence for feedback occurs each time a print request is made, resulting in extra load on the apparatus and an increase in the operation cost.
JP-A-2003-312061 discloses a method of directly limiting the sum total of color components using a multi-dimensional LUT. In such a method, however, if the number of lattice points is five in a LUT corresponding to four-dimensional input/output of CMYK, as many as 54×4=2500 pieces of data are required.
Use of such a large-scaled multi-dimensional LUT leads to an increase in cost because a large memory space is consumed when the LUT is installed in hardware. If an LUT needs to be again set from the beginning, for example, as the image processing mode is switched between print pages, etc., the time required for transferring the LUT data increases and thus an obstacle to speeding up is presented. A similar problem also occurs in JP-A-2000-25274 and JP-A-2000-343761 because a multi-dimensional LUT is used.
To use a multi-dimensional LUT, calculation (or signal processing) load of interpolation calculation also occurs. For example, if interpolation calculation of comparatively simple multiple linear interpolation is performed for finding, when the interpolation value is calculated from the vertices of eight peripheral grids as in FIG. 3 in JP-A-2000-343761, as many multiplications as (number of input channels)×(number of vertices)×(number of output channels) become necessary.
In this case, the number of input channels is four for CMYK, the number of vertices is eight (interpolation using eight peripheral vertices in black point 11 in FIG. 3 in JP-A-2000-343761), and the number of output channels is four (CMYK) and therefore the necessary number of multiplications becomes 128. Thus, in hardware implementation, the logic scale grows and in software implementation, the computation amount becomes an obstacle to speeding up.
JP-A-2003-125225 discloses a method of feeding back so that the total amount becomes a preset value or less for each input of each color signal. In this method, iteration of processing caused by feeding back occurs for each pixel and thus the method is disadvantageous to speeding up of real-time image processing. In contrast, the saturation and contrast of an image would be able to be improved by performing processing of increasing the total amount in the range in which the total amount does not exceed the preset value, but the method in JP-A-2003-125225 cannot be applied to the processing of increasing the total amount.
JP-A-2005-33348 discloses a method of increasing the toner total amount within the range of a preset value for improving the image quality under a special condition to perform 100% UCR in gray in such a meaning. Such processing has the advantage that the gray balance does not change if subtle balance change occurs in the gray level characteristic of C, M and Y.
In JP-A-2005-33348, however, as seen in FIG. 7, correction coefficient f (k) is used in the range of f (k)>1 and thus if it is also used for limiting the toner total amount in usual four-color separation to mainly use the range of f (k)<1, the f (k) range and resolution and memory and circuit scale saving become difficult to be compatible.
In JP-A-2005-33348, for gray scale correction (γ correction) provided at the later stage of four-color separation as seen in FIG. 1, if it is released to the user directly as a density (or brightness) adjustment parameter, it becomes necessary to provide change caused by user adjustment as a margin in addition to the fluctuation of the engine for the limitation value of the toner total amount. Thus, only the color material less the margin than the essential capability of the printer engine can be used and the color reproduction range is narrowed.